#include <stdio.h>

int fa(int n){
	if(n <= 0) return -1;
	int i=0,x = 1,y = 1;//x 是n-1项，y是第n项
	for(i=0;i<n-2;i++){
		y = y + x; //y是第n+1项
		x = y - x;//x是第n项
	}	
	return y;
}

int main(){
	int i;
	printf("请输入一个数：\n");
	scanf("%d",&i);
	printf("%d\n",fa(i));
}
